Master cylinder

ABSTRACT

A master cylinder includes a housing and a piston movable therein, which is sealed relative to a pressure chamber by way of a sealing element fixed on the housing. The pressure chamber is connectable to an unpressurized supply chamber by at least one transverse bore provided in the piston, wherein the transverse bore has a slot-shaped configuration and opens into at least one groove provided on the outer surface of the piston.

BACKGROUND OF THE INVENTION

The present invention relates to a master cylinder, in particular forhydraulic automotive vehicle brake systems.

In U.S. Pat. No. 5,187,934, for example, a master cylinder is disclosedwhich includes a piston movable therein, which is sealed relative to apressure chamber by way of a sealing element fixed on the housing, thepressure chamber being connectable to an unpressurized supply chamber bymultiple transverse bores provided in the piston. It is a disadvantagein the prior art master cylinder that transverse bores in the pistonmust be deburred in order to avoid damage to the sealing element whenoverriding the transverse bores. Also, many small-diameter bores arerequired to permit a high amount of fluid flow without causing damage tothe sealing element. A large-diameter bore would permit a high amount offluid flow, however, would increase the risk of destroying the sealingelement and cause a large lost travel.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a generictandem master cylinder which permits a high amount of fluid flow throughthe transverse bores without increasing the risk of destroying thesealing element.

According to the present invention, this object is achieved by a mastercylinder having transverse bores that open into at least one grooveprovided on the outer surface of the piston. The groove is used toincrease the flow cross-section. The groove, or a plurality of grooves,can extend either in an axial direction, or over part of the peripheryof the outer piston surface. The formation of burrs at the port of thetransverse bore is of little interest because the port is placed in thegroove. Possible burrs do not get into contact with the sealing element,thus, wear or damage to the sealing element is effectively prevented.

Because of its symmetric shape, a circumferential groove is particularlyeasy to manufacture. A groove of this type also permits a maximumexpansion of the flow cross-section and, thus, a slow velocity of flowof the hydraulic medium along with a high amount of fluid flow.

A slight inclination of the lateral surfaces of the groove with respectto the outer surface of the piston causes little impairment of thesealing element when it is overridden by the groove. An inclination ofapproximately 30° has proved especially suitable because the lateralsurfaces are sufficiently flat in this case, i.e., the transition issufficiently smooth, and an appropriately large fluid flow cross-sectionis achieved in addition. However, equally good properties are achievedwith a discrepancy from this value of about ±15°. It is particularlyfavorable to configure the cross-section of the groove as a V, with thelegs of the V being at an obtuse angle relative to each other. Thisachieves good properties and a simple manufacture.

The expanded cross-section achieved by the groove of the presentinvention necessitates only a small number of transverse bores with acorrespondingly large cross-section. The cross-section of the transversebore may be large because, at its port on the outer surface of thepiston, the sealing element is not impaired due to the groove of thepresent invention. Two transverse bores, which lie preferably on oneaxis, can be manufactured particularly easily, for example, by drillingin one operation. Four bores also bear this advantage. It is evensimpler to manufacture one single transverse bore.

A particularly large cross-section and a small axial extension of thetransverse bore with respect to the piston axis is achieved when thetransverse bore has a slotted shape. A transverse bore of this type iseasy to manufacture in punching or injection-molding operations.Transverse bores with an oval cross-section, for example, are alsopossible.

When the supply chamber is provided in a guide element which guides thepiston, the guide element has radial recesses on the side facing thepiston according to the present invention.

The recesses are separated from each other in a circumferentialdirection by at least one web. The piston can abut directly on the web,ensuring an especially proper guide despite the partial recesses in theguide element. In the reset position of the piston, the recesses areconnected to the groove according to the present invention and, thus,provide a pressure fluid connection between the pressure chamber and thesupply chamber regardless of the respective angular position of thepiston relative to the axis of the master cylinder. It is not necessaryto fix the orientation of angles. A sufficient flow cross-section isensured due to the depth of the recesses. At least one recess and oneweb, which is almost circumferential, is provided to this effect. Or,correspondingly, two or more recesses are provided which are separatedfrom each other by two or more webs.

When the recess extends in an axial direction up to a sealing elementwhich abuts on the guide element, the sealing element can be wetted byhydraulic medium. The durability of the sealing element is increased bythis wetting action, especially when the sealing element is adjacent tothe outer piston surface.

Further advantages of the present invention can be seen in the followingdescription, making reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a cross-sectional view of a master cylinder according to thepresent invention.

FIGS. 2a, 2b are enlarged cross-sectional views of FIG. 1 proximate theleft guide element.

FIGS. 3a, 3b are enlarged cross-sectional views of FIG. 1 proximate theright guide element.

DETAILED DESCRIPTION OF THE DRAWINGS

The master cylinder 1 in FIG. 1 is the tandem master cylinder of anautomotive vehicle brake system including two pistons 2, 3. The pistons2, 3 are guided in corresponding guide elements 4, 5 and sealed bysealing elements 6, 6' or 7, 7'.

The sealing elements 6, 6' are axially positioned by the guide element 4and a sleeve 8. The sealing elements 7, 7' are axially fixed in positionby the guide element 5 and a bushing 9. Interposed between the sleeve 8and the guide element 5 is a washer 10 which also contributes to fixingthe sealing element 7 axially in position.

A washer 11 is arranged between a bushing 9 and a closure cap 13. Thewasher 11 has resilient portions 11' which bear against the closure cap13 and urge the bushing 9 against the guide element 5. Thus, acompensation is provided for (possibly) slightly different axialpositions of the closure cap 13 relative to the bushing 9 or the guideelement 7. Small axial tolerances of the individual components can alsobe compensated this way.

The closure cap 13 closes an opening 15 of the master cylinder 1 fromwhich a stepped axial bore 12 extends to an opening 14. A backward partof the piston 2 projects from the opening 14. A piston rod 16 abuts onthe piston 2 to apply an actuating force F₁ to the piston 2.

A plastic tube 40 has one or more radial projections 39 at its front endwhich is disposed in a pressure chamber 22, 23. The radial projections39 can move into abutment on a step of sleeve 8, with the result thatthe piston 2 is prevented from dropping out of the master cylinder 1.

The pistons 2 and 3, respectively, have a smooth, cylindrical outersurface 32 and 33, respectively, and include a hollow chamber 34 and 35,respectively. A spring assembly 17, interposed between the pistons 2 and3, comprising a hollow dowel pin 17', clamping sleeves 18 and 19 and aspring 20, exerts an axial force on piston 3, on displacement of piston2, to displace piston 3. When no actuating force F₁ is applied to thepiston 2, piston 2 is reset by the force of spring 20. Piston 3 is resetby the force of the spring 21 which is supported on the closure cap 13.

A primary pressure chamber 22 is arranged between the pistons 2 and 3,and a secondary pressure chamber 23 is arranged between the piston 3 andthe closure cap 13. The pressure chambers 22 and 23, respectively, areconnnected to different brake circuits of an automotive vehicle by theschematically indicated pressure connections 24 and 25, respectively.

In the reset condition of the pistons 2 and 3, respectively, thepressure chambers 22 and 23 are connected to the supply chambers 28 and29, respectively, by way of transverse bores 26 and 27 in thecorresponding pistons. Supply chambers 28 and 29 are provided in theguide elements 4 and 5, respectively. Instead of a transverse bore 26,an axial groove 26' may be provided which is indicated as an alternativein the half of the piston 2 illustrated above the axis A of mastercylinder 1 shown in dotted lines. The supply chambers 28 and 29,respectively, are connected to an unpressurized supply reservoir (notshown) by way of reservoir ports 30 and 31, respectively.

Prior to the operation of the master cylinder 1, the pistons 2, 3 adopttheir reset initial position shown. The pressure chambers 22, 23 areconnected to the unpressurized compensating reservoir (not shown), andthe hydraulic medium in the chambers is unpressurized.

To operate the master cylinder 1, an actuating force F₁ is applied tothe piston 2 by way of the piston rod 16, piston 2 thereby moving to theleft, as viewed in the drawing. Simultaneously, the piston 3 isdisplaced to the left by the biassed spring 20 of the spring assembly17. The transverse bores 26, 27 or, if provided, the corresponding axialgrooves 26' override the sealing elements 6', 7', with the result thatthe hydraulic connection between the primary pressure chamber 22 and thesupply chamber 28 and between the secondary pressure chamber 23 and thesupply chamber 29 is interrupted. Further displacement of the pistons 2,3 to the left causes an increase in pressure in the pressure chambers22, 23. Hydraulic medium is conducted through the pressure ports 24, 25to operate the connected wheel brakes (not shown).

When the actuating force F₁ decreases, the pistons 2, 3 are displaced tothe right by the pressure prevailing in the pressure chambers 22, 23 andby the force of springs 20, 21. If necessary, hydraulic medium can beaspirated from the supply chambers 28, 29 through the external sealinglip of the sealing elements 6', 7' into the corresponding pressurechambers 22, 23. When the pistons 2, 3 have assumed their initialposition, there is again a direct connection between the pressurechambers 22, 23 and the corresponding supply chambers 28, 29 by way ofthe transverse bores 26, 27.

Return movement of the pistons, or of one of the pistons, to thecorresponding initial position can also be effected by a pressureincrease in the pressure chamber 22 or 23, respectively.

When a tandem master cylinder according to the present invention is usedin a brake system with control (ABS control, TCS control, drivingstability control, or the like), a pressure increase of this type canoccur during a corresponding control operation.

The transverse bores 26, 27 or, respectively, the axial groove 26', openinto a circumferential groove 100, 101 according to the presentinvention. This arrangement is shown on an enlarged scale in thefollowing Figures.

FIG. 2a shows an enlarged cross-sectional view of FIG. 1 proximate theguide element 5 and the part of piston 3 above the axis A, while FIG. 2bshows the respective lower part.

The supply bores configured as transverse bores 27 extend from thehollow chamber 35 of the piston 3 to the groove 101. Groove 101 has aV-shaped cross-section having lateral surfaces 103, 105 which areinclined at an acute angle with respect to the outer piston surface 107.In FIG. 2a, the supply chamber 29 is comprised of a radial bore 109 andan axial bore 11 which are connected to the groove 101 by way of arecess 113 when the piston 3 adopts its reset position as illustrated.The recess 113 extends in an axial direction not entirely until thesealing elements 7 and 7', respectively. When the piston 3 is displacedto the left, there is no slot between the outer piston surface 107 andthe guide element 5, into which the sealing elements 7, 7' could bepressed which are acted upon by the pressure that prevails in thepressure chamber 22 and 23, respectively. A supporting disc 117 preventsthe sealing element 7' from entering into the axial bore 111. The slotwhich is produced by the groove 101 when the piston 3 moves to the leftis so small in size that the sealing element 7' is not impaired.

In the section shown in FIG. 2b, the supply chamber is configured as asubstantially radially extending transverse bore 109' which opensproximate the circumferential groove 101 when the piston 3 has adoptedits reset position. A recess, corresponding to the recess 113, toconnect the groove 101 to the supply chamber 29 is not required in thiscase. FIG. 2b shows a web 115 which is provided on either side of therecess 113 in an almost completely circumferential manner.

In FIG. 3a, the area of the guide element 4 and the piston 2 above theaxis A can be seen. In this case, however, the transverse bore 26instead of the axial groove 26' is illustrated. FIG. 3b shows thecorresponding half below axis A. The transverse bores 26 open into thecircumferential groove 100 having lateral surfaces 102, 104 which aredesigned similarly to FIGS. 2a, 2b.

The supply chamber 28 is comprised of a radial bore 108 and an axialbore 110 and is connected to an unpressurized compensating reservoir(not shown) by way of the supply bore 118. The radial bore 108 opensinto a recess 112 of the guide element 4 which extends from the area ofthe circumferential groove 100 in an axial direction until the area ofthe sealing element 6 (not shown in FIG. 3a), as can be seen in FIG. 1.On the one hand, bore 108 provides a connection between the supplychamber 28 and the circumferential groove 100 in the reset condition ofthe piston 2, similarly as described with respect to FIG. 2a. On theother hand, bore 108 is used to wet the sealing element 6 with hydraulicmedium.

FIG. 3b shows the web 114 which is arranged between the recesses 112.The circumferential groove 100 is connected to the supply chamber 28 byway of recesses 112 adjacent to web 114, which cannot be seen in thiscross-section.

Upon operation, the piston 2 is moved to the left, and groove 100 leavesthe area of the guide element 4 and overrides the sealing element 6'.Subsequently, pressure is built up in the pressure chamber 22, with theresult that the sealing lips of the sealing element 6' are pressedagainst the outer piston surface 106 and the stepped bore 12. Thesupporting disc 116 is urged against the axial bore 110, and the sealingelement 6' abuts safely on the supporting disc 116.

The connection between the pressure chamber 22 and the unpressurizedreservoir in the nonactuated piston end position, as shown in FIG. 3a,is provided by the circumferential groove 100 according to the presentinvention. At least one transverse bore 26 opens inwards into groove100. In the mentioned end position, the groove 100 is connected to theunpressurized reservoir by way of the recess 112 in the guide element 4on the housing, or by way of a transverse bore exiting at the samelocation, similar to the transverse bore 109' in FIG. 2b. Thus, apressure fluid connection is ensured when the piston 2 is reset. Becausepressure-loaded sealing elements 6', 7, 7', especially those having asleeve-type design, are destroyed in large slots by extrusion, thecircumferential grooves 100, 101 are rated as small slots. The smallslots, nevertheless, have a large cross-section because they have acircumferential design. To permit a large cross-section of theconnection, without increasing the extension of the transverse bore 26in an axial direction of the master cylinder, the transverse bore 26 inFIG. 3b is configured as a slot having its longer axis arrangedvertically to the drawing plane.

The present invention discloses that the transverse bores 26 of a mastercylinder 1 including a plunger piston 2 open into a circumferentialgroove 100 provided in the outer surface 106.

We claim:
 1. A master cylinder including a housing and a piston movabletherein, which is sealed relative to a pressure chamber by way of asealing element fixed on the housing, the pressure chamber beingconnectable to an unpressurized supply chamber by at least onetransverse bore provided in the piston,wherein at least one groove isprovided on the outer surface of the piston, the transverse bore openinginto the groove and having a slot-shaped configuration to increase flowof fluid through the transverse bore.
 2. The master cylinder as claimedin claim 1, wherein the groove is a circumferential groove.
 3. Themaster cylinder as claimed in claim 1, wherein the lateral surfaces ofthe groove are inclined only slightly with respect to the outer surfaceof the piston.
 4. The master cylinder as claimed in claim 1, wherein thegroove has a cross-section in the shape of a V having legs which are atan obtuse angle relative to each other.
 5. The master cylinder asclaimed in claim 1, wherein two to four transverse bores having anappropriately large cross-section are provided in the piston.
 6. Themaster cylinder as claimed in claim 1, wherein one transverse borehaving an appropriately large cross-section is provided in the piston.7. The master cylinder as claimed in claim 1, wherein the supply chamberis provided in a guide element in which the piston is guided, the guideelement including on the piston side radial recesses which are separatedfrom each other in a circumferential direction by at least one web. 8.The master cylinder as claimed in claim 7, wherein the recess extendsaxially adjacent to a sealing element, wherein said sealing elementabuts on the guide element.
 9. A master cylinder including a housing andat least one piston movable therein, which is sealed relative to apressure chamber by way of a sealing element fixed on the housing, thepressure chamber being connectable to an unpressurized supply chamber byat least one transverse bore provided in the piston,wherein at least onegroove is provided on the outer surface of the piston, the transversebore opening into the groove and having a slot-shaped configuration;wherein the groove has a cross-section in the shape of a generally Vhaving two legs, said legs each being at an obtuse angle relative toeach other.
 10. The master cylinder as claimed in claim 9, wherein the Vcross-section is generally symmetrical.